1. Field of the Invention
This invention relates to so-called power steering "for parking." While the standard power steering systems provide a boost torque to the steering wheel torque exerted by the driver on the steering column, whatever the speed of the vehicle may be, the so-called power steering systems for parking exert their boost only at reduced speeds.
The principle of the boost "for parking" is already applied on some hydraulic and electric power steering systems. Its application to electric power steering is illustrated by publication FR 2,248,702. According to this publication, the driving unit of the booster system is put into operation by a circuit depending on the speed of the vehicle, the driving being cut off when the vehicle exceeds a certain speed.
Many electric power steering systems, such as the one described in publication FR 2,573,380, comprise an input shaft, an output shaft connected mechanically to the input shaft by a torsion bar, a torque sensor detecting the torque exerted on the torsion bar, and a dc motor exerting a boost torque on the output shaft, in response to signals from the torque sensor.
According to publication FR 2,573,380, the control signal of the motor can advantageously consist of a rotor voltage having a value proportional to the amplitude of the steering wheel torque imposed on the input shaft. A torque sensor particularly intended for electric power steering is described in publication FR 2,630,699. This sensor measures the relative torque existing between a primary steering shaft and a secondary steering shaft, and emits corresponding signals to a servomotor driving a secondary shaft as a function of the signals. It is also known to add to the performance by making the boost ratio vary as a function of the speed of the vehicle. This is applied in the device which is the object of publication FR 2,248,702, according to which the electric booster motor meets the variable conditions of movement of the vehicle, transmitted in particular by a speed sensor.